Washington (CNN)US investigators wiretapped former Trump campaign chairman Paul Manafort under secret court orders before and after the election, sources tell CNN, an extraordinary step involving a high-ranking campaign official now at the center of the Russia meddling probe.

Some of the intelligence collected includes communications that sparked concerns among investigators that Manafort had encouraged the Russians to help with the campaign, according to three sources familiar with the investigation. Two of these sources, however, cautioned that the evidence is not conclusive.

Special counsel Robert Mueller’s team, which is leading the investigation into Russia’s involvement in the election, has been provided details of these communications.

A secret order authorized by the court that handles the Foreign Intelligence Surveillance Act (FISA) began after Manafort became the subject of an FBI investigation that began in 2014. It centered on work done by a group of Washington consulting firms for Ukraine’s former ruling party, the sources told CNN.

The surveillance was discontinued at some point last year for lack of evidence, according to one of the sources.

The FBI then restarted the surveillance after obtaining a new FISA warrant that extended at least into early this year.

Sources say the second warrant was part of the FBI’s efforts to investigate ties between Trump campaign associates and suspected Russian operatives. Such warrants require the approval of top Justice Department and FBI officials, and the FBI must provide the court with information showing suspicion that the subject of the warrant may be acting as an agent of a foreign power.

It is unclear when the new warrant started. The FBI interest deepened last fall because of intercepted communications between Manafort and suspected Russian operatives, and among the Russians themselves, that reignited their interest in Manafort, the sources told CNN. As part of the FISA warrant, CNN has learned that earlier this year, the FBI conducted a search of a storage facility belonging to Manafort. It’s not known what they found.

The conversations between Manafort and Trump continued after the President took office, long after the FBI investigation into Manafort was publicly known, the sources told CNN. They went on until lawyers for the President and Manafort insisted that they stop, according to the sources.

It’s unclear whether Trump himself was picked up on the surveillance.

The White House declined to comment for this story. A spokesperson for Manafort didn’t comment for this story.

Manafort previously has denied that he ever “knowingly” communicated with Russian intelligence operatives during the election and also has denied participating in any Russian efforts to “undermine the interests of the United States.”

The FBI wasn’t listening in June 2016, the sources said, when Donald Trump Jr. led a meeting that included Manafort, then campaign chairman, and Jared Kushner, the President’s son-in-law, with a Russian lawyer who had promised negative information on Hillary Clinton.

That gap could prove crucial as prosecutors and investigators under Mueller work to determine whether there’s evidence of a crime in myriad connections that have come to light between suspected Russian government operatives and associates of Trump.

The FBI interest in Manafort dates back at least to 2014, partly as an outgrowth of a US investigation of Viktor Yanukovych, the former Ukrainian president whose pro-Russian regime was ousted amid street protests. Yanukovych’s Party of Regions was accused of corruption, and Ukrainian authorities claimed he squirreled millions of dollars out of the country.

Investigators have spent years probing any possible role played by Manafort’s firm and other US consultants, including the Podesta Group and Mercury LLC, that worked with the former Ukraine regime. The basis for the case hinged on the failure by the US firms to register under the US Foreign Agents Registration Act, a law that the Justice Department only rarely uses to bring charges.

All three firms earlier this year filed retroactive registrations with the Justice Department.

It hasn’t proved easy to make a case.

Last year, Justice Department prosecutors concluded that there wasn’t enough evidence to bring charges against Manafort or anyone of the other US subjects in the probe, according to sources briefed on the investigation.

The FBI and Justice Department have to periodically seek renewed FISA authorization to continue their surveillance.

As Manafort took the reins as Trump campaign chairman in May, the FBI surveillance technicians were no longer listening. The fact he was part of the campaign didn’t play a role in the discontinued monitoring, sources told CNN. It was the lack of evidence relating to the Ukraine investigation that prompted the FBI to pull back.

Manafort was ousted from the campaign in August. By then the FBI had noticed what counterintelligence agents thought was a series of odd connections between Trump associates and Russia. The CIA also had developed information, including from human intelligence sources, that they believed showed Russian President Vladimir Putin had ordered his intelligence services to conduct a broad operation to meddle with the US election, according to current and former US officials.

The FBI surveillance teams, under a new FISA warrant, began monitoring Manafort again, sources tell CNN.

The court that oversees government snooping under FISA operates in secret, the surveillance so intrusive that the existence of the warrants only rarely become public.

For that reason, speculation has run rampant about whether Manafort or others associated with Trump were under surveillance. The President himself fueled the speculation when in March he used his Twitter account to accuse former President Barack Obama of having his “wires tapped” in Trump Tower.

The Justice Department and the FBI have denied that Trump’s own “wires” were tapped.

While Manafort has a residence in Trump Tower, it’s unclear whether FBI surveillance of him took place there.

Manafort has a home as well in Alexandria, Virginia. FBI agents raided the Alexandria residence in July.

The FBI also eavesdropped on Carter Page, a campaign associate that then candidate Trump once identified as a national security adviser. Page’s ties to Russia, including an attempt by Russian spies to cultivate him, prompted the FBI to obtain a FISA court warrant in 2014.